1. Technical Field
The present invention relates to an optical fiber for use in the field of optical communication applications which input, into the optical fiber, signal light having a relatively high optical energy.
2. Related Art
According to applications such as passive optical network (PON) systems, signals are transmitted via a single optical fiber to a location near subscribers and split at the location appropriately in accordance with the number of the subscribers. When this technique is utilized, the further the splitting location is positioned from the base station, or the larger the number of the subscribers is, the higher optical energy the base station needs to input into the optical fiber.
Recent development of amplifiers such as erbium doped fiber amplifiers (EDFAs) has increasingly enabled high optical energy to be input into the optical fiber. Generally speaking, however, when signal light having a high energy is input into an optical fiber, a phenomenon called stimulated Brillouin scattering (SBS) occurs. This phenomenon reduces the amount of optical energy that is actually transmitted. As mentioned, for example, in the non-Patent Document 1, a threshold value (mW) of the optical energy to cause the SBS is proportional to the effective cross-section (Ae) of the optical fiber, and inversely proportional to the Brilliouin gain coefficient gB and effective interacting length (Le).
Here, the SBS occurs in the following manner. A periodic density distribution is formed within the optical fiber due to acoustic phonons, and functions as gratings, to scatter the signal light. Since the acoustic phonons also move, the scattered light has a frequency slightly lower than the original signal light because of the Doppler effect. The interference between the scattered light and signal light further excites the acoustic phonons, thereby increasing the scattering intensity.
When the signal light has an intensity equal to or lower than the threshold value to cause the SBS, the scattering has little influence on the signal light. However, when the intensity of the signal light exceeds the threshold value, the scattering intensity radically increases. Once the intensity of the signal light reaches a certain level, the increase in the intensity of the signal light input into the optical fiber only raises the scattered light, and does not enhance the intensity of the actually transmitted signal. On the contrary, such an increase raises noise, thereby degrading the signal.
In view of the above, a variety of methods have been proposed to raise the threshold value to cause the SBS.
Here, the spectrum of the scattered light differs depending on the composition of the materials and distortion of the optical fiber used. It has been found that the broader the spectrum of the scattered light is, the higher the threshold value to cause the SBS becomes. According to Patent Document 1, for example, a method is disclosed to raise the threshold value to cause the SBS by varying the diameter of the core, the refractive index and distortion in the longitudinal direction of the optical fiber. Patent Document 2 discloses a method to raise the threshold value to cause the SBS by varying the density of the dopant such as fluorine in the longitudinal direction of the optical fiber in the step of forming part of the core and clad portions when a preform for an optical fiber is manufactured by using a two-step method. Patent Document 3 discloses a similar method of varying the dopant density in the longitudinal direction. Patent Document 4 discloses a method to simultaneously vary both the diameter of the core and the relative refractive index difference in the longitudinal direction. According to all of the methods mentioned above, the values of one or more parameters are varied in the longitudinal direction of the optical fiber.
In addition to the above-mentioned methods, Patent Document 5 discloses the following method. A plurality of annular regions are formed in the vicinity of the boundary between the core and clad portions. The annular regions each have a small thickness, and are uniform in the longitudinal direction. In each of the annular regions, a dopant having a different coefficient of thermal expansion (CTE) and a different viscosity is doped. The annular regions are adjusted so as not to affect the transmission characteristics which are determined by the refractive index distribution. In this manner, a distortion distribution is formed in the radial direction in the optical fiber, in order to reduce the SBS.
Non-Patent Document 1: “Optical Fiber Telecommunications IIIA”, Academic Press, page 200
Patent Document 1: Japanese Patent No. 2584151
Patent Document 2: Japanese Patent No. 2753426
Patent Document 3: Unexamined Japanese Patent Application Publication No. H09-301738
Patent Document 4: Unexamined Japanese Patent Application Publication No. H10-96828
Patent Document 5: U.S. Pat. No. 6,542,683
When a method of varying one or more characteristic parameters of an optical fiber in the longitudinal direction of the optical fiber is used, remarkable effects can be obtained by varying the parameters in short intervals of equal to or shorter than approximately 1 km. However, low-cost methods of manufacturing optical fibers based on large preforms have difficulties in varying the parameters in such short intervals, considering that the drawing rate is as high as 1 km/min or higher and, when a large preform is used, a preform having a length of as short as approximately 1.5 mm to 5 mm is sufficient to manufacture an optical fiber of 1 km by drawing.
On the other hand, the method in which thin annular regions are formed so as to be arranged in the radial direction of the optical fiber by using a plurality of dopants is very difficult to be practiced because the dopants disperse in the radial direction during the manufacturing process of the optical fiber. In addition, an optical fiber manufactured by using this method suffers from a larger loss than a normal optical fiber, and has a small value for Le. Therefore, this method has difficulties in achieving desired effects. The optical fiber manufactured by means of this method poses another drawback that an even higher optical energy needs to be input.
An advantage of some embodiments of the present invention is to provide an optical fiber which can be manufactured at low costs, has a higher threshold value to cause the SBS, and enables signal light having a high energy to be input. This object is achieved by combining the features recited in the independent claims. The dependent claims define further effective specific example of the present invention.